1. Field of the Invention
The present invention relates generally to the field of fiber optics and in particular to improved few-mode fiber designs for spatial multiplexing.
2. Background Art
Data traffic over fiber optic networks continues to grow exponentially. In order to meet this demand, multiplexing techniques have been developed that allow a plurality of separate data streams to share the same optical fiber, thereby significantly increasing per-fiber traffic.
Current research and development in the optical fiber industry have focused on dense wavelength-division multiplexing (DWDM), which is a multiplexing technique, in which a plurality of data channels are assigned to respective wavelengths within an operating bandwidth. The data channels are combined onto the fundamental (LP01) mode of a single-mode fiber for transmission, and are separated back into separate channels when they reach their destination.
In a DWDM-based transmission system, the total capacity within a given amplifier bandwidth is limited by spectral efficiency, which is a measure of how tightly separate wavelengths can be spaced for transmission purposes at a given data rate when ultimately limited by nonlinear effects in the fiber. Spectral efficiency can be increased using increasingly complex methods, such as the use of higher-order modulation schemes, but such methods have diminishing returns and the modest improvement cannot keep pace with the exponential growth of bandwidth demand. It is anticipated that, over the next 10 to 15 years, the spectral efficiency of DWDM in a single-mode fiber will approach its theoretical limit.
One promising approach for increasing per-fiber capacity is space division multiplexing (SDM), in which a plurality of data channels within a single optical fiber are provided by a respective plurality of fiber cores, or by a respective plurality of optical signal modes guided by the fiber. SDM-based techniques have the potential to significantly increase per-fiber transmission capacity beyond the limits imposed by non-linearity in DWDM-based systems.
Because of the fundamental differences between SDM and prior technologies, SDM has proven to be technologically challenging, requiring the development of new types of optical fibers and associated devices.